An aim of targeted drug therapy is to use monoclonal antibodies (MAbs) for the specific delivery of toxic agents to human cancers. Conjugates of tumor-associated MAbs and suitable toxic agents have been developed, but have had mixed success in the therapy of cancer, and virtually no application in other diseases, such as autoimmune diseases. The toxic agent is most commonly a chemotherapeutic drug, although particle-emitting radionuclides, or bacterial or plant toxins have also been conjugated to MAbs, especially for the therapy of cancer (Sharkey and Goldenberg, C A Cancer J. Clin. 2006 July-August; 56(4):226-243).
The advantages of using MAb-chemotherapeutic drug conjugates are that (a) the chemotherapeutic drug itself is structurally well defined; (b) the chemotherapeutic drug is linked to the MAb protein using very well defined conjugation chemistries, often at specific sites remote from the MAb antigen binding regions; (c) MAb-chemotherapeutic drug conjugates can be made more reproducibly than chemical conjugates involving MAbs and bacterial or plant toxins, and as such are more amenable to commercial development and regulatory approval; and (d) the MAb-chemotherapeutic drug conjugates are orders of magnitude less toxic systemically than radionuclide MAb conjugates.
Early work on protein-drug conjugates indicated that a drug preferably is released in its original form, once it has been internalized into a target cell, for the protein-chemotherapeutic drug conjugate to be a useful therapeutic. Trouet et al. (Proc. Natl. Acad. Sci. USA 79:626-629 (1982)) showed the advantage of using specific peptide linkers, between the drug and the antibody, which are cleaved lysosomally to liberate the intact drug. MAb-chemotherapeutic drug conjugates prepared using mild acid-cleavable linkers, such as those containing a hydrazone, were developed, based on the observation that the pH inside tumors was often lower than normal physiological pH (Willner et al., U.S. Pat. No. 5,708,146; Trail et al. (Science 261:212-215 (1993)). The first approved MAb-drug conjugate, Gemtuzumab Ozogamicin, incorporated a similar acid-labile hydrazone bond between an anti-CD33 antibody, humanized P67.6, and a potent calicheamicin derivative. Sievers et al., J Clin Oncol. 19:3244-3254 (2001); Hamann et al., Bioconjugate Chem. 13: 47-58 (2002). In some cases, the MAb-chemotherapeutic drug conjugates were made with reductively labile hindered disulfide bonds between the chemotherapeutic drugs and the MAb (Liu et al., Proc Natl Acad Sci USA 93: 8618-8623 (1996)).
Yet another cleavable linker involves cathepsin B-labile dipeptide spacers, such as Phe-Lys or Val-Cit, similar to the lysosomally labile peptide spacers of Trouet et al. containing from one to four amino acids, which additionally incorporated a collapsible spacer between the drug and the dipeptide (Dubowchik, et al., Bioconjugate Chem. 13:855-869 (2002); Firestone et al., U.S. Pat. No. 6,214,345 B1; Doronina et al., Nat. Biotechnol. 21: 778-784 (2003)). The latter approaches were also utilized in the preparation of an immunoconjugate of camptothecin (Walker et al., Bioorg Med Chem. Lett. 12:217-219 (2002)). Another cleavable moiety that has been explored is an ester linkage incorporated into the linker between the antibody and the chemotherapeutic drug. Gillimard and Saragovi have found that when an ester of paclitaxel was conjugated to anti-rat p75 MAb, MC192, or anti-human TrkA MAb, 5C3, the conjugate was found to exhibit target-specific toxicity. Gillimard and Saragovi, Cancer Res. 61:694-699 (2001).
The conjugates of the instant invention possess greater efficacy, in many cases, than unconjugated or “naked” antibodies or antibody fragments, although such unconjugated targeting molecules have been of use in specific situations. In cancer, for example, naked antibodies have come to play a role in the treatment of lymphomas (CAMPATH® and RITUXAN®), colorectal and other cancers (ERBITUX® and AVASTIN®), breast cancer (HERECEPTIN®), as well as a large number now in clinical development (e.g., epratuzumab). In most of these cases, clinical use has involved combining these naked, or unconjugated, antibodies with other therapies, such as chemotherapy or radiation therapy.
A variety of antibodies are also in use for the treatment of autoimmune and other immune dysregulatory diseases, such as tumor necrosis factor (TNF) and B-cell (RITUXAN®) antibodies in arthritis, and are being investigated in other such diseases, such as the B-cell antibodies, RITUXAN® and epratuzumab, in systemic lupus erythematosus and Sjögren's syndrome, as well as juvenile diabetes and multiple sclerosis. Naked antibodies are also being studied in sepsis and septic shock, Alzheimer's disease, and infectious diseases.
There is a need to develop more potent immunoconjugated antibodies against B cell diseases, such as cancer, autoimmune disease, immune dysfunction disease, type 1 and type 2 diabetes. There is a further need to develop more effective antibody conjugates with intracellularly cleavable linkers. In the case of delivering drug/toxin or radionuclide conjugates, this can be accomplished by direct antibody conjugation or by indirect methods, referred to as pretargeting, where a bispecific antibody is used to target to the lesion, while the therapeutic agent is secondarily targeted by binding to one of the arms of the bispecific antibody that has localized at the site of the diseased cell (Goldenberg et al., J Clin Oncol. 2006 Feb. 10; 24(5):823-34; Goldenberg et al., J Nucl Med. 2008 January; 49(1):158-63).
Because signaling pathway redundancies can result in lack of response to a single antibody, diverse strategies to use combination therapy with antibodies that bind to different epitopes or different antigens on the same target cell have been proposed. Combinations such as anti-CD20 and anti-CD22 (Stein et al., Clin Cancer Res 2004, 10:2868-2878), anti-CD20 and anti-HLA-DR (Tobin et al., Leuk Lymphoma 2007, 48:944-956), anti-CD20 and anti-TRAIL-R1 (Maddipatla et al., Clin Cancer Res 2007, 13:4556-4564), anti-IGF-1R and anti-EGFR (Goetsche et al., Int J Cancer 2005, 113:316-328), anti-IGF-1R and anti-VEGF (Shang et al., Mol Cancer Ther 2008, 7:2599-2608), or trastuzumab and pertuzumab that target different regions of human EGFR2 (Nahta et al., Cancer Res 2004, 64:2343-2346) have been evaluated preclinically, showing enhanced or synergistic antitumor activity in vitro and in vivo.
The first clinical evidence of an apparent advantage of combining two antibodies against different cancer cell antigens involved the administration of rituximab (chimeric anti-CD20) and epratuzumab (humanized anti-CD22 antibody) in patients with non-Hodgkin lymphoma (NHL). The combination was found to enhance anti-lymphoma efficacy without a commensurate increase in toxicity, based on 3 independent clinical trials (Leonard et al., J Clin Oncol 2005, 23:5044-5051). Although these results are promising, a need exists in the field for more effective antibody-based combination therapies.